I've got a problem with going downhill as well. Also, turning to the right in too tight a circle. If you've fallen off before when doing something (in my case, the 2 above), it really messes with your nerves. I came off going downhill when my brakes failed once (read: threw myself off to avoid going into a busy road) and quite recently was turning right and the front wheel just slipped out underneath.
Psychological damage, that's what it is!
Every time I go to do those things, physically I feel like I'm re-creating the circumstances where I ended up hurting myself before. Even though a fear of turning right is so ridiculous all I can think of is Zoolander. I'd love to know the 'cure' for this - but otherwise, I'd say if your wife is happy to accept her own limitations, it's so much more relaxing to cycle in a way which makes you feel safe and secure on the bike. Feeling terrified something is about to slip or go wrong is really unpleasant. I plan my routes around this exact thing - avoid big downhills, avoid going too fast round right turns. Although admittedly I'm not competitive, just cycling to commute and for pleasure!